Terminology
Chemical Looping Combustion method or CLC: In the text hereafter, what is referred to as CLC (Chemical Looping Combustion) is an oxidation-reduction or redox looping method on an active mass. It can be noted that, in general, the terms oxidation and reduction are used in connection with the respectively oxidized or reduced state of the active mass. The oxidation reactor is the reactor where the redox mass is oxidized and the reduction reactor is the reactor where the redox mass is reduced. During reduction of the redox mass, the fuel can be either totally oxidized, producing CO2 and H2O, or partly oxidized, producing synthesis gas CO and H2.
Prior Art
Upgrading of heavy petroleum cuts that cannot be distilled under atmospheric conditions is often sought after.
A known way of directly upgrading petroleum cuts that cannot be distilled under atmospheric conditions (340° C.+cut) consists in burning them to produce energy. The combustion of this type of fuels however raises the problem of the capture of the CO2 emitted in the fumes, harmful to the environment.
The CLC (Chemical Looping Combustion) method consists in using redox reactions of an active mass so as to split the combustion reaction into two successive reactions. A first reaction of oxidation of the active mass with air or a gas acting as the oxidizer allows, due to the exothermic character of the oxidation, a hot gas to be obtained whose energy can then be used. A second reaction of reduction of the active mass thus oxidized, by means of a reducing compound, then allows a reusable active mass and a gaseous mixture essentially comprising carbon dioxide and water to be obtained. This technique thus allows the carbon dioxide to be isolated in a gaseous mixture practically free of oxygen and nitrogen.
U.S. Pat. No. 5,447,024 describes a CLC method comprising an active mass reduction reactor using a reducing gas and an oxidation reactor allowing the active mass to be restored in its oxidized state by means of an oxidation reaction with humidified air. The circulating fluidized-bed technology is used to allow continuous change of the active mass from its oxidized state to its reduced state.
The active mass that alternately changes from its oxidized form to its reduced form, and vice versa, follows a redox cycle. It can be noted that, generally, the terms oxidation and reduction are used in connection with the respectively oxidized or reduced state of the active mass. The oxidation reactor is the reactor where the redox mass is oxidized and the reduction reactor is the reactor where the redox mass is reduced.
Thus, in the reduction reactor, the active mass (MxOy) is first reduced to the state MxOy−2n−m/2, by means of a hydrocarbon CnHm, which is correlatively oxidized to CO2 and H2O, according to reaction (1), or possibly to mixture CO+H2 depending on the proportions used.CnHm+MxOy→nCO2+m/2H2O+MxOy−2n−m/2  (1)
In the oxidation reactor, the active mass is restored to its oxidized state (MxOy) on contact with air according to reaction (2), before returning to the first reactor.MxOy−2n−m/2+(n+m/4)O2→MxOy  (2)
The efficiency of the circulating fluidized bed CLC method is based to a large extent on the physico-chemical properties of the redox active mass. The reactivity of the redox pair(s) involved and the associated oxygen transfer capacity are parameters that influence the dimensioning of the reactors and the rates of circulation of the particles. The life of the particles depends on the mechanical strength of the particles and on their chemical stability. In order to obtain particles usable for this method, the particles involved generally consist of a redox pair or a series of redox pairs selected from among CuO/Cu, Cu2O/Cu, NiO/Ni, Fe2O3/Fe3O4, FeO/Fe, Fe3O4/FeO, MnO2/Mn2O3, Mn2O3/Mn3O4, Mn3O4/MnO, MnO/Mn, Co3O4/CoO, CoO/Co, and of a binder providing the required physico-chemical stability.
Many studies have been conducted with gas feeds (essentially methane) and solid feeds, and they have shown the feasibility of chemical looping combustion for this type of feeds.
For gas feeds, a direct reactive chemistry occurs between the oxidized solid particles and the fuel. As for solid feeds, they first have to be gasified (to synthesis gas CO and H2) to allow conversion thereof on contact with the redox mass. Gasification thus represents the temporally limiting stage. Therefore, in order to increase the residence time of the solid feed, the unconverted feed from the reduction reactor is separated from the redox mass (through density difference between the solid feed and the redox mass via a solid/solid separator—U.S. Pat. No. 2,896,709) to be recycled.
Patent application WO-2008/036,902 A2 describes the CLC method applied in a general way to various feeds: gaseous, liquid and solid. The use of heavy liquid feeds, refining products that have the particular feature of not being distillable under atmospheric conditions, is not considered.
Using liquid fuels generates additional implementation difficulties that are not encountered for gaseous or solid feeds, i.e. vaporization of the feed on contact with the redox masses and coke formation around the redox particles. Unlike solid feeds, it is not possible to consider separating the unburnts (coke settled on the redox mass) from the redox mass for recycle to the reduction reactor (too slight particle density and size differences). The residence time of the liquid feed in this reactor is advantageously strictly controlled to guarantee oxidation of the vaporized feed, as well as gasification of the coke formed and oxidation thereof, in order to obtain the desired oxidation level in a single pass.
We have discovered that the combustion of heavy liquid feeds can be carried out in a Chemical Looping Combustion type process, by contacting the liquid feed with a redox mass in a fluidized bed, in order to upgrade the heavy liquid petroleum fractions resulting from refining, while avoiding the problem of CO2-rich fumes.